Conventionally, fabrication of a photoresist pattern begins by forming a resist layer on a semiconductor substrate, possibly followed by heating the resist layer to, for example, evaporate any solvent. The desired pattern is then transferred to the resist layer by exposing the resist layer using, for example, an immersion lithography tool. After an optional post-exposure bake is performed, the exposed resist layer is developed using a developer rinse. A subsequent deionized (DI) water rinse then removes the unwanted portions of the resist layer, leaving the resist pattern on the substrate. Spin drying may be subsequently employed to remove any remaining droplets of deionized water. However, while this process for fabricating a photoresist pattern is well known and practiced, it does have its shortcomings.
For example, the resist pattern may have a high contact angle, which can induce developer watermarks. During the spin drying step, the DI water is spun away from the wafer center towards the wafer edge. Consequently, the DI water drops become smaller and smaller and can ultimately become so small that they adhere to the resist surface. That is, if the van der Waals force holding the DI water droplet to the resist surface becomes greater than the centrifugal force urging the droplet away from the wafer, then the droplet will remain on the resist surface. Moreover, because the DI water is utilized to rinse away the exposed (or unexposed) resist, the DI wafer can retain soluble polymers and other chemicals from the exposed (or unexposed) resist. Consequently, if the DI water droplet is allowed to evaporate on the resist surface, the polymers and other chemicals in the droplet result in a stain on the resist surface, and can therefore become a defect source. For example, if the stain covers a patterned hole in the resist, it would result in a blind contact.
Thus, while some kinds of polymers may exhibit good lithographic performance, many may also have a high contact angle which can induce developer water mark defects. In contrast, other kinds of polymers have a lower contact angle, and therefore decrease the occurrence of developer watermark defects, but these polymers often do not exhibit acceptable lithographic performance.
Nonetheless, there are chemical treatments which can reduce the contact angle after the developing DI rinse is performed. However, such treatment chemicals can become residue that also gets captured in the DI droplets, which can dissolve the resist pattern and also become a defect source.
Moreover, if the polymer contact angle is too low, the immersion fluid utilized during an immersion lithography exposure process can also form water drop and defect residue on the resist surface. Such water drop residue and defect residue can, thus, also become a defect source, often referred to as an immersion watermark and immersion fall-on defect.